1. Field of the Invention
This invention relates to a motor drive unit for driving an electric motor (hereinafter simply referred to as motor) such as a fan motor provided in a computer, a switchboard, and a printer, and more particularly to a motor drive unit having a motor lock-up detection circuit.
2. Description of the Related Art
FIG. 4 is a schematic diagram showing a general arrangement of a motor drive unit equipped with a conventional motor lock-up detection circuit. Shown in FIG. 4 is a 2-phase motor 1 for rotating a fan. A resistor 2a, a Hall device 2, and a resistor 2b are connected in series between a power supply voltage Vdd and the ground. The rotational condition of the motor 1 is detected by the Hall device 2. The output signal of the Hall device 2 is fed to a semiconductor integrated circuit (IC) 100, which is adapted to drive drive-transistors 9a and 9b. 
A first-stage amplifier 3 amplifies the output of the Hall device 2, and outputs a rotational signal A. A mid-stage amplifier 4 further amplifies the rotational signal A and supplies it to a control circuit 5. The control circuit 5 generates a 2-phase drive signal based on the signal received from the mid-stage amplifier 4 and a reference signal. This drive signal is supplied to the drive-transistors 9a and 9b. Element 8 is a diode.
These components constitute a feedback loop for driving the motor 1 with a 2-phase half-wave driving signal in response to the rotational signal A. In a steady state, the motor 1 maintains a rotation at a substantially constant speed determined by the characteristics of the feedback loop.
An anomalous condition can take place where the fan is temporarily stopped or locked up by, for example, an obstacle accidentally hitting the fan. Under such lock-up condition, the rotational signal A will not change. As a consequence, the control circuit 5 remains locked up in a fixed output status. Thus, the motor 1 is either continually supplied with electric power or no longer supplied with any electric power from the control circuit 5.
If electric power is continually supplied to the stopped motor 1, excessive current will flow through it, thereby resulting in abnormal heating and eventual destruction of the motor 1. On the other hand, when no electric power is supplied to the motor 1, it cannot restore its rotational motion even after the obstacle is removed. In any event, accidental stopping of the motor entails adverse consequences.
To cope with such disadvantage, motor drive units are normally provided with an automatic return signal generation circuit 6 and a capacitor 7 for detecting lock-up condition and fulfilling automatic restoration function. Although the capacitor 7 collaborates with the automatic return-signal generation circuit 6, it is (usually) provided as an external component to be mounted outside the IC 100. However, when the electric capacity of the capacitor 7 is small, it may be built in the IC 100. For example, it may be included in the automatic return signal generation circuit 6.
The first-stage amplifier 3, mid-stage amplifier 4, control circuit 5, and automatic return signal generation circuit 6 are built in the IC 100, and connected to external elements via associated pins P1 and P2 (for hole signals), pin P3 (for capacitor 7), and pins P4 and P5 (for drive signals), as shown.
The automatic return signal generation circuit 6 monitors the rotational condition of the motor 1 based on the rotational signal A received. When it is detected that the rotation of the motor 1 has stopped, the circuit 6 starts and continues generating an automatic return signal E until the motor 1 resumes rotation. The automatic return signal E alternately assumes an ON state for a predetermined period (referred to as startup trial (re-startup) period) and an OFF state for another predetermined period (referred to as dormant period) as determined by the automatic return signal generation circuit 6 and the capacitor 7.
In the event that the motor 1 is stopped, the automatic return signal E is used instead of the control signal of the amplifier 4, in the control circuit 5 to restart the motor 1. For example, startup trial operation of about 0.5 second is repeatedly tried in each of startup trial periods, interlaced with dormant periods of about 3 second each, until the motor 1 restores its rotational motion.
As a result, unless the motor 1 is broken in the accident, it can recover a rotational motion as soon as a rotatable condition is restored. Incidentally, the lengths of a startup trial period and a dormant period are determined in accordance with the characteristics of the motor in question.
Referring to FIG. 5, there is shown an exemplary arrangement of a conventional automatic return signal generation circuit 6. The capacitor 7 has a capacitance of 1 μF, for example, which generates a triangular or sawtooth charging and discharge voltage signal (hereinafter referred to as charging-discharging voltage signal) C as it is charged and discharged. A first constant current circuit 63 is adapted to supply charging current Ic1 of 3 μA, for example, to the capacitor 7. A second constant current circuit 64 is adapted to supply discharging current Id of 3.5 μA for example from the capacitor 7.
A comparison circuit 65 provides a HIGH (H) level comparison output D when the voltage of the charging-discharging voltage signal C inputted thereto exceeds a given operational threshold level of, for example, about 2.5 V, and provides a LOW (L) level comparison output D when the voltage of the charging-discharging voltage signal C is below a predetermined return threshold level, which is, for example, about, 1 V. A second switching circuit SW2 turns on or off the discharging current from the second constant current circuit 64 according to the level of the comparison output D. These elements constitute an oscillation circuit. In the example shown herein, the oscillatory charging-discharging voltage signal C thus obtained is an asymmetrical triangular wave signal that takes about 0.5 seconds to rise and about 3 seconds to fall.
In response to a rotational signal A, the pulse generation circuit 61 generates a pulse signal B having the same frequency as the rotational signal A. Upon receipt of a pulse signal B, the first switch SW1 instantly discharges the charged capacitor 7.
Because of these switches being connected to the oscillation circuit, when the motor 1 is in steady rotation, a pulse signal B is generated with a period of cycle in accord with the short period of the rotational signal A, thereby repeating discharging the capacitor 7 with that short period. Accordingly, while the motor 1 is in steady rotation, the oscillation of the charging-discharging voltage signal C is suppressed, so that the signal C turns out to be a sawtooth wave that oscillates slightly about “0 level”. As a result, the comparison output D of the comparison circuit 65 remains at “L level”.
On the other hand, if the motor 1 is stopped, the rotational signal A will not vary any longer, which causes the pulse signal B to be stopped, which in turn causes the oscillation circuit to start its own oscillation as stated above. That is, the charging-discharging voltage signal C becomes an asymmetrical triangular wave signal that rises for about 0.5 seconds and falls for about 3 seconds. The comparison output D of the comparison circuit 65 also becomes a similar pulse wave having the same period of oscillation as the charging-discharging signal C. Thus, whether the motor 1 is in the rotational state or in a locked-up condition or not can be detected from the difference in waveform of the charging-discharging voltage signal C. In this sense a so-called lock-up detection function can be fulfilled by the charging-discharging circuit.
A waveform shaping circuit 66 reshapes the waveform of the charging-discharging voltage signal C to provide a pulsed automatic return signal E. This automatic return signal E has alternating “H” startup trial period (each lasting about 0.5 second) and “L” dormant periods (each lasting about 3 seconds). This automatic return signal E is outputted after detection of the stopping of the motor 1 and until the motor 1 restores its rotational motion. So-called automatic return function is achieved in this way by generating such automatic return signal E.
In the automatic return signal generation circuit 6 shown in FIG. 5, current levels of the first and second constant current circuits 63 and 64, respectively, cannot be made very high. Therefore, in order to obtain required lengths of startup trial period and dormant period, it is necessary to make large the capacitance of the capacitor 7. In an effort to reduce the capacitance of the capacitor, Japanese Patent Application Laid Open H7-131995 (hereinafter referred to as Patent Document 1) discloses an automatic return signal generation circuit 6 equipped with an oscillation circuit for instantly discharging the capacitor by means of the comparator 65, a counter for counting the output of the comparator 65, and a comparator for generating an automatic return signal E based on the comparison of the count of the counter with a predetermined value.
It is noted that this prior art automatic return signal generation circuit has a limitation that each of the startup trial periods and the dormant periods has a fixed length (e.g. Startup trial period is about 0.5 sec and dormant period is about 3 sec) as set in accordance with the characteristics of the motor 1 used. Hence, the ratio of the two periods (defined to be the ratio of the startup trial period divided by the dormant period) is fixed to a predetermined value.
The startup trial period, dormant period, and the ratio are optimized for the type of the motor 1 to be used. Thus, there is a problem with prior art automatic return signal generation circuits that an IC 100 must have an automatic return signal generation circuit 6 optimized for that particular type of the motor used.
Although the first and second constant current circuits 63 and 64, respectively, can be modified to provide variable current levels so that the startup trial period and the dormant period, and hence the ratio thereof, can be varied, such modification raises the cost of the IC 100.